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VARIABLE FREQUENCY DRIVE
CHAPTER 1: INTRODUCTION In today‟s industrial and commercial world, variable frequency drives (also known as VFD’s, Frequency Drives, AC motor controllers, and Inverters) are becoming a more and more important method of speed control. They are used for all but the most demanding speed control applications. Large rolling mills are among the few places where DC motors and controls are still used. One of the reasons for this is that AC motors and their associated controls are much more reliable than the DC motors and the associated controls that to a large extent they have replaced. In current market VFD‟s ranging from ¼ hp to 30,000 hp are available for use. In 1960 the first inverter were made and had limited applications due to small size and reliability in 1980‟s when high power transistor were available, larger inverters were made and many more applications opened up. Reliability and Mean Time between Failures (MTBF) was still a problem. All of these earlier devices used linear amplifiers and controls for their basic operation. Small potentiometers and dip switches were used to set their operating characteristics. In the 1990‟s digital controls began to be used more and more in Inverters. Solid state devices were developed that allowed higher voltage and current ratings. Inverters are used in larger motors. Micro processors made the Inverter a versatile device. For many applications, the Inverter can be removed from a packing box, wired to a motor, and turned on and operated without additional set up. The motors that are usually controlled by VFD‟s are induction motors. A three phase induction motor is one of the simplest power conversion devices ever made. It has one moving part. If the motor has ball bearings, then an induction motor with ball bearings does have more than one moving part. VFD‟s are very simple, and hence very reliable. They have a winding on the stator, or part that stands still, and a winding on the rotor, or the part that turns. When voltage is applied to the stator, a voltage is induced (Hence – induction motor) in the stator coil. This causes a current to flow in both the stator and the rotor. The design of the motor is such that the magnetic fields of the two currents act against each other to cause a force on the rotor and make it rotate.
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VARIABLE FREQUENCY DRIVE
Efficiencies of over 90 % and power factors of over 80 % are common at full load. Some larger motors have power factors of up to 90 % when fully loaded. However, lightly loaded AC induction motors typically have low efficiency and low power factor.
CHAPTER 2: VARIABLE FREQUENCY DRIVE Motors are designed to run at a constant speed. However, motor drive systems are often operated at part or variable load. In particular, fans and pumps can have highly irregular load profiles. This means, the motors on these systems either run at constant speed by ing the excess capacity, or use some form of capacity regulation such as dampers, valves, or inlet guide vanes, all of which are very inefficient. System output can be controlled by adjusting the speed of the motor using one of different types of Adjustable Speed • Variable Frequency Drives (VFDs) Motion control is required in large number of electrical and domestic applications like rolling mills, fans, paper machines, textile mills, pumps, robots etc. System employed for motion control is called drives. Drives employing electric motors are known as electrical drives. SOURCE POWER MODULATOR SENSING UNIT CONTROL
UNIT
MOTOR LOAD
BLOCK DIAGRAM OF AN ELECTRIC DRIVE
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VARIABLE FREQUENCY DRIVE
SOURCE POWER MODULATOR SENSING UNIT CONTROL
UNIT
LOAD VFD MOTOR
BLOCK DIAGRAM OF AN ELECTRIC DRIVE WITH VFD In this block diagram, load is usually a machinery to accomplish a given task, e.g. fan, pumps, robots, drills etc. A motor having speed-torque characteristics and capabilities compatible to load requirement is chosen. Power modulator are used to modulate flow of power from the source to motor in a manner that required by the load, during transient operations, converts electrical energy of the source in the form of suitable to motor and also selects the modes of operation of the motor. Power modulator is as desired, it may also generate commands for the protection of power modulator and motor. Input command signal adjust the operating point of drive, forms an input control unit. In this basic block diagram a variable speed drive can be placed so as to adjust the speed of the motor as required by the load. VFDs of higher efficiencies are easier to control, require less maintenance, and have become the drive of choice in the majority of applications. In addition, speed control is generally the most energy-efficient flow control technique because it requires the least amount of energy to meet the given load. VFDs have widespread acceptance in the industry therefore we will focus our attention on these drives. Within the VFD family, there are several types available, each with its own practical application. All perform the job of controlling the motor speed by varying electrical voltage and frequency in response to an
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VARIABLE FREQUENCY DRIVE
electric from the end use. Depending on the application, VFDs, when applied correctly, can reduce energy consumption by more than 50 percent. Induction motors find applications in low to high power applications and synchronous motor drives are employed in high power and medium power drives. The permanent magnet synchronous motor and brushless dc motors are being considered for replacing dc servo motors for fractional hp range. As the trend exits, applications of ac drives will continue to grow. The speed of an Ac motor is determined for the most part by two factors. The applied frequency and number of poles. N=120f / P Where: N=RPM f=frequency P=number of poles Some motor such as in typical paddle fan have the capability to switch poles in and out to control speed. In most cases however, the number of poles is constant and the only way to vary the speed is to change the applied frequency changing the frequency is the primary function of VFD. We must that the impedance of the motor is determined by inductive reactance of winding and given by X L= 2∏fL X L= inductance in ohms f = line frequency L = inductance This means that if the frequency applied to the motor is reduced, the reactance and therefore impedance of the motor is reduced. In order to keep current under control we must lower the applied voltage to the motor as the frequency is reduced. This is where we get the phrase volt
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per hertz. The most common method of controlling the applied voltage and frequency is with a pulse width modulated „PWM‟ technique. With this method, a dc voltage is applied to the motor windings in time controlled pulses in order to achieve current that approximates a sine wave for the desired frequency. IGBTs are the latest technology and often the ability to switch the PWM very fast. This allows several thousand pulses to be applied in one cycle of the applied motor frequency. More pulses in a given cycle result in a smoother current wave form and better performance.
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CHAPTER 3: OPERATION OF VFD Understanding the basic principles behind VFD operation requires understanding the three basic sections of the VFD: the rectifier, dc bus, and inverter. The voltage on an alternating current (ac) power supply rises and falls in the pattern of a sine wave (see Figure 1). When the voltage is positive, current flows in one direction; when the voltage is negative, the current flows in the opposite direction. This type of power system enables large amounts of energy to be efficiently transmitted over great distances.
2 1 0 -
90
180
270
360
1
-2 Fig 1. AC SINE WAVE
The rectifier in a VFD is used to convert incoming ac power into direct current (dc) power. One rectifier will allow power to through only when the voltage is positive. A second rectifier will allow power to through only when the voltage is negative. Two rectifiers are required for each phase of power. Since most large power supplies are three phase, there will be a minimum of 6 rectifiers used (See fig 2). Appropriately, the term “6 pulse” is used to describe a drive with 6 rectifiers. A VFD may have multiple rectifier sections, with 6 rectifiers per section, enabling a VFD to be “12 pulse,” “18 pulse,” or “24 pulse.”
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Fig 2. VFD (existing technology) Rectifiers may utilize diodes, controlled
rectifiers
(SCR),
silicon
or
transistors to rectify power. Diodes are the simplest device and allow power to flow any time voltage is of the proper polarity. Silicon con-trolled rectifiers include a gate circuit that enables a microprocessor to control when the power may begin to flow, making this type of rectifier useful for solid-state starters as well. Transistors include a gate circuit that enables a microprocessor to open or close at any time, making the transistor the most useful device of the three. A VFD using transistors in the rectifier section is said to have an “active front end.”
Fig 3. Basic
block diagram of
VFD
After
the
power flows through
the rectifiers it is stored on a dc bus. The dc bus contains capacitors to accept power from the rectifier, store it, and later deliver that power through the inverter section. The dc bus may also contain inductors, dc links, chokes, or similar items that add inductance, thereby
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smoothing the incoming power supply to the dc bus. The final section of the VFD is referred to as an “inverter.” The inverter contains transistors that deliver power to the motor. The “Insulated Gate Bipolar Transistor” (IGBT) is a common choice in modern VFDs. The IGBT can switch on and off several thousand times per second and precisely control the power delivered to the motor. The IGBT uses a method named “pulse width modulation” (PWM) to simulate a current sine wave at the desired frequency to the motor. Motor speed (rpm) is dependent upon frequency. Varying the frequency output of the VFD controls motor speed: Speed (rpm) = frequency (hertz) x 120 / no. of poles
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CHAPTER 4: TYPES OF VFD VOLTAGE SOURCE INVERTER (VSI): Variable frequency and variable voltage supply for IM control can be obtained from VSI. VSI allows a variable frequency to be obtained from a dc supply. It can be operated as a stepped wave inverter or pulse width modulated inverter. When supply is dc, variable dc input is obtained by connecting a chopper between dc supply and inverter. When supply is ac, dc input voltage is obtained by connecting a controlled rectifier between supply ac and inverter. A large electrolytic capacitor is connected in dc link to make inverter operation independent of rectifier or chopper and to filter out the harmonics in dc link voltage. Harmonics are reduced, low frequency harmonics are eliminated, associated losses are reduced and smooth motion is obtained at low speeds also when inverter is operated as pulse width modulated inverter. Since output voltage can be controlled by PWM, no arrangement is required for variation of input dc voltage, inverter can be directly connected when supply is dc and through a diode rectifier when supply is ac.
VSI induction motor drives V = m Vd / 2√ 2 Where m is modulation index.
CURRENT SOURCE INVERTER (CSI):
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In current source inverter, inverter behaves as current source due to the presence of large inductance in dc link. For a given speed, torque is controlled by varying dc link current by changing the inverter voltage. Therefore, when supply is ac, a controlled rectifier is connected between the supply and inverter and when supply is dc, a chopper is interposed between the supply and inverter. The maximum value of dc output voltage of fully controlled rectifier and chopper are chosen so that the motor terminal voltage saturates at rated value.
CSI induction motor drives
COMPARISON OF CSI AND VSI: The relative advantages of CSI and V SI are ●
CSI is more reliable than VSI because 1
Condition of two devices in the same leg due to commutation failure does not lead to sharp rise of current through them.
2 ●
it has inherent protection against short circuit across motor terminals
Because of large inductance in dc link and large inverter capacitors, CSI drives has higher cost, weight and volume, lower speed range and slower dynamic response.
●
The CSI drive is not suitable for multimotor drives. Hence, each motor is fed from its own inverter and rectifier. A single converter can be used to feed a number of VSI
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motor systems connected in parallel. A single VSI can similarly feed a number of motors connected in parallel.
CHAPTER 5: ENERGY SAVING WITH VFD Rising energy prices create a need for energy-efficient motor control solutions. To help reduce energy losses, process engineers are turning to variable frequency drives (or adjustable speed drives) as an alternative to fixed speed controllers and throttling devices such as dampers and valves. Advancements in drive technology, careful selection of the hardware and power system configuration, and intelligent motor control strategies produce improved operating performance, control capability and energy savings. Things to consider when choosing a motor control solution include peak-demand charges, operating at optimized efficiency, power factor, isolation transformer cost and losses, regeneration capabilities, synchronous transfer options and specialized intelligent motor control energy-saving features.
Optimize Power Usage: Centrifugal loads offer the greatest potential for energy savings by using variable frequency drives (VFDs) to control speed. Energy consumption in centrifugal fan and pump applications follows the affinity laws, which means that flow is proportional to speed, pressure is proportional to the square of speed, and horsepower is proportional to the cube of speed. That means if an application only needs 80 percent flow, the fan or pump will run at 80 percent of rated speed and only requires 50 percent of rated power. In other words, reducing speed by 20 percent requires only 50 percent of the power. ●
Beat peak-demand charges VFDs also affect peak-demand electricity prices that the utility charges to companies
that exceed a preset limit, such as when industrial motors started across-the-line draw large peaks of current. VFDs help reduce the peaks by supplying the power needed by the specific application, and gradually ramping the motor up to speed to reduce the current drawn
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●
Power Factor Power factor and how it affects displacement and harmonic distortion is another
important consideration in drive selection. Drives that are near-unity true power factor translate to reduced energy use. ●
Regeneration
Some VFD applications enable s not only to save energy, but to regenerate power, which can be routed back to the system or sold to a utility for additional revenue.. ●
Multiple Motors drive
Another way to reduce energy costs is the synchronous by method which uses only one VFD to start and synchronize multiple motors by transferring a load from one source to another by matching the voltage waveform frequency, amplitude and phase relation between the two sources. Using a VFD to start a motor, bring it up to speed and then synchronize it, causes a reduction in full-load current and optimizes the process. ●
Extra Energy-Saving Potential
Intelligent motor control solutions including high-efficiency variable frequency drives are an important part of an energy savings program. But not all drives have the same capabilities. Software features and programmability can further contribute to a drive‟s energy savings potential by reducing inrush current requirements. Programmability - s can program their VFD to adjust the total acceleration time and current limit and adjust the speed to the load requirement. Current limit on drives is normally set between 105 and 110 percent, whereas using the across-the-line starting method produces current limits of approximately 650 percent.
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SGCTs – Advances in power semiconductor switches like SGCTs (symmetrical gatecommutated thyristors) are designed for high-voltage operation and ensure the lowest switching and conduction losses while maintaining a high switching frequency. Power Optimization – Power optimizing features optimize the power usage when operating fans and pumps by adjusting the required voltage to the application. This reduces losses for improved motor and drive efficiency. Communication software – Software features enable torque limit and integrated architecture through communication connectivity between the drives, starters and soft starters for greater control and optimizatio
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VARIABLE FREQUENCY DRIVE
CHAPTER 6: APPLICATIONS The three major areas where drives and motor are applied are Variable torque application – When a low torque is needed at low speed and high torque at high speed. These applications are generally centrifugal loads such as fans, pumps and blowers. These are good targets of energy savings. Constant power application – When a high torque is required at low speed and low torque at high speed. It includes machine tools and traction. Constant torque application- When the same amount of torque is required at low speed as at high speed. Power is directly proportional to speed. They include conveyors, mixers, screw feeders, extruders and positive displacement pumps. Lifts – Energy saving when regenerative breaking is involved. Transport- Where all the vehicles, train, ship etc involves motor, VFDs are used for speed control.
The latest developments in technology and successful development of electronic drives (AC drives) for cage motors have resulted in the following benefits: 1
Availability of full load torque from standstill
2
Absence of torque fluctuations at low speed.
3
Ability to hold a set speed, regardless of load torque variation
4
Ability to control the rate of increase & decrease of speed
5
Dynamic response.
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CHAPTER 7: ADVANTAGES AND LIMITATIONS
Advantages: ●
Soft starting
●
Precise speed and torque control
●
Wide speed control torque
●
High reliability and availability
●
Low audible noise
●
Capability of speed reversal / regenerative breaking
●
Flux optimization
●
Power loss ride through
●
Energy saving
Limitations: ●
AC drives not reliable as dc drives
●
Cost of AC drives are more
●
Presence of harmonics may heat the motor
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●
Separate filter circuits may be required to reduce harmonics
CONCLUSIONS: For precise process control and wide speed range, to provide good running and transient performances variable frequency drives are natural choice. The report describes functioning of such a drive. It is very essential to know the applications of them. The consumer is using them enormous. Hence, this paper concludes that variable frequency drive plays an in important role in speed control of ac electric motors and also VFDs are simple, reduced thermal stress on motor, most energy efficient, reduced noise levels, energy savings, soft starting and stopping are the key features of the VFD.
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VARIABLE FREQUENCY DRIVE
BIBLIOGRAPHY: 1
Gopal k.Dubey, Fundamentals of electrical drives, , Narosa Publishing house, II edition
2 3 4
Bimal K Bose, Modern power electronics and AC drives M. H Rashid , Power electronics, Pearson India, II edition Bimal K Bose,The past, present and future of Power Electronics, IEEE industrial magazine vol.3
5
Marcel Dekker ,Solid state AC motor controls, ,New York
6
Carrier drive technology Syracus, Variable frequency drive technology, New York
7
www.wikipedia.org
8
nptel.iitk.ac.in
9
www.ab.com
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CHAPTER 1: INTRODUCTION In today‟s industrial and commercial world, variable frequency drives (also known as VFD’s, Frequency Drives, AC motor controllers, and Inverters) are becoming a more and more important method of speed control. They are used for all but the most demanding speed control applications. Large rolling mills are among the few places where DC motors and controls are still used. One of the reasons for this is that AC motors and their associated controls are much more reliable than the DC motors and the associated controls that to a large extent they have replaced. In current market VFD‟s ranging from ¼ hp to 30,000 hp are available for use. In 1960 the first inverter were made and had limited applications due to small size and reliability in 1980‟s when high power transistor were available, larger inverters were made and many more applications opened up. Reliability and Mean Time between Failures (MTBF) was still a problem. All of these earlier devices used linear amplifiers and controls for their basic operation. Small potentiometers and dip switches were used to set their operating characteristics. In the 1990‟s digital controls began to be used more and more in Inverters. Solid state devices were developed that allowed higher voltage and current ratings. Inverters are used in larger motors. Micro processors made the Inverter a versatile device. For many applications, the Inverter can be removed from a packing box, wired to a motor, and turned on and operated without additional set up. The motors that are usually controlled by VFD‟s are induction motors. A three phase induction motor is one of the simplest power conversion devices ever made. It has one moving part. If the motor has ball bearings, then an induction motor with ball bearings does have more than one moving part. VFD‟s are very simple, and hence very reliable. They have a winding on the stator, or part that stands still, and a winding on the rotor, or the part that turns. When voltage is applied to the stator, a voltage is induced (Hence – induction motor) in the stator coil. This causes a current to flow in both the stator and the rotor. The design of the motor is such that the magnetic fields of the two currents act against each other to cause a force on the rotor and make it rotate.
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VARIABLE FREQUENCY DRIVE
Efficiencies of over 90 % and power factors of over 80 % are common at full load. Some larger motors have power factors of up to 90 % when fully loaded. However, lightly loaded AC induction motors typically have low efficiency and low power factor.
CHAPTER 2: VARIABLE FREQUENCY DRIVE Motors are designed to run at a constant speed. However, motor drive systems are often operated at part or variable load. In particular, fans and pumps can have highly irregular load profiles. This means, the motors on these systems either run at constant speed by ing the excess capacity, or use some form of capacity regulation such as dampers, valves, or inlet guide vanes, all of which are very inefficient. System output can be controlled by adjusting the speed of the motor using one of different types of Adjustable Speed • Variable Frequency Drives (VFDs) Motion control is required in large number of electrical and domestic applications like rolling mills, fans, paper machines, textile mills, pumps, robots etc. System employed for motion control is called drives. Drives employing electric motors are known as electrical drives. SOURCE POWER MODULATOR SENSING UNIT CONTROL
UNIT
MOTOR LOAD
BLOCK DIAGRAM OF AN ELECTRIC DRIVE
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VARIABLE FREQUENCY DRIVE
SOURCE POWER MODULATOR SENSING UNIT CONTROL
UNIT
LOAD VFD MOTOR
BLOCK DIAGRAM OF AN ELECTRIC DRIVE WITH VFD In this block diagram, load is usually a machinery to accomplish a given task, e.g. fan, pumps, robots, drills etc. A motor having speed-torque characteristics and capabilities compatible to load requirement is chosen. Power modulator are used to modulate flow of power from the source to motor in a manner that required by the load, during transient operations, converts electrical energy of the source in the form of suitable to motor and also selects the modes of operation of the motor. Power modulator is as desired, it may also generate commands for the protection of power modulator and motor. Input command signal adjust the operating point of drive, forms an input control unit. In this basic block diagram a variable speed drive can be placed so as to adjust the speed of the motor as required by the load. VFDs of higher efficiencies are easier to control, require less maintenance, and have become the drive of choice in the majority of applications. In addition, speed control is generally the most energy-efficient flow control technique because it requires the least amount of energy to meet the given load. VFDs have widespread acceptance in the industry therefore we will focus our attention on these drives. Within the VFD family, there are several types available, each with its own practical application. All perform the job of controlling the motor speed by varying electrical voltage and frequency in response to an
Department of Electrical and Electronics,SDM IT
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VARIABLE FREQUENCY DRIVE
electric from the end use. Depending on the application, VFDs, when applied correctly, can reduce energy consumption by more than 50 percent. Induction motors find applications in low to high power applications and synchronous motor drives are employed in high power and medium power drives. The permanent magnet synchronous motor and brushless dc motors are being considered for replacing dc servo motors for fractional hp range. As the trend exits, applications of ac drives will continue to grow. The speed of an Ac motor is determined for the most part by two factors. The applied frequency and number of poles. N=120f / P Where: N=RPM f=frequency P=number of poles Some motor such as in typical paddle fan have the capability to switch poles in and out to control speed. In most cases however, the number of poles is constant and the only way to vary the speed is to change the applied frequency changing the frequency is the primary function of VFD. We must that the impedance of the motor is determined by inductive reactance of winding and given by X L= 2∏fL X L= inductance in ohms f = line frequency L = inductance This means that if the frequency applied to the motor is reduced, the reactance and therefore impedance of the motor is reduced. In order to keep current under control we must lower the applied voltage to the motor as the frequency is reduced. This is where we get the phrase volt
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per hertz. The most common method of controlling the applied voltage and frequency is with a pulse width modulated „PWM‟ technique. With this method, a dc voltage is applied to the motor windings in time controlled pulses in order to achieve current that approximates a sine wave for the desired frequency. IGBTs are the latest technology and often the ability to switch the PWM very fast. This allows several thousand pulses to be applied in one cycle of the applied motor frequency. More pulses in a given cycle result in a smoother current wave form and better performance.
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VARIABLE FREQUENCY DRIVE
CHAPTER 3: OPERATION OF VFD Understanding the basic principles behind VFD operation requires understanding the three basic sections of the VFD: the rectifier, dc bus, and inverter. The voltage on an alternating current (ac) power supply rises and falls in the pattern of a sine wave (see Figure 1). When the voltage is positive, current flows in one direction; when the voltage is negative, the current flows in the opposite direction. This type of power system enables large amounts of energy to be efficiently transmitted over great distances.
2 1 0 -
90
180
270
360
1
-2 Fig 1. AC SINE WAVE
The rectifier in a VFD is used to convert incoming ac power into direct current (dc) power. One rectifier will allow power to through only when the voltage is positive. A second rectifier will allow power to through only when the voltage is negative. Two rectifiers are required for each phase of power. Since most large power supplies are three phase, there will be a minimum of 6 rectifiers used (See fig 2). Appropriately, the term “6 pulse” is used to describe a drive with 6 rectifiers. A VFD may have multiple rectifier sections, with 6 rectifiers per section, enabling a VFD to be “12 pulse,” “18 pulse,” or “24 pulse.”
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VARIABLE FREQUENCY DRIVE
Fig 2. VFD (existing technology) Rectifiers may utilize diodes, controlled
rectifiers
(SCR),
silicon
or
transistors to rectify power. Diodes are the simplest device and allow power to flow any time voltage is of the proper polarity. Silicon con-trolled rectifiers include a gate circuit that enables a microprocessor to control when the power may begin to flow, making this type of rectifier useful for solid-state starters as well. Transistors include a gate circuit that enables a microprocessor to open or close at any time, making the transistor the most useful device of the three. A VFD using transistors in the rectifier section is said to have an “active front end.”
Fig 3. Basic
block diagram of
VFD
After
the
power flows through
the rectifiers it is stored on a dc bus. The dc bus contains capacitors to accept power from the rectifier, store it, and later deliver that power through the inverter section. The dc bus may also contain inductors, dc links, chokes, or similar items that add inductance, thereby
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smoothing the incoming power supply to the dc bus. The final section of the VFD is referred to as an “inverter.” The inverter contains transistors that deliver power to the motor. The “Insulated Gate Bipolar Transistor” (IGBT) is a common choice in modern VFDs. The IGBT can switch on and off several thousand times per second and precisely control the power delivered to the motor. The IGBT uses a method named “pulse width modulation” (PWM) to simulate a current sine wave at the desired frequency to the motor. Motor speed (rpm) is dependent upon frequency. Varying the frequency output of the VFD controls motor speed: Speed (rpm) = frequency (hertz) x 120 / no. of poles
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VARIABLE FREQUENCY DRIVE
CHAPTER 4: TYPES OF VFD VOLTAGE SOURCE INVERTER (VSI): Variable frequency and variable voltage supply for IM control can be obtained from VSI. VSI allows a variable frequency to be obtained from a dc supply. It can be operated as a stepped wave inverter or pulse width modulated inverter. When supply is dc, variable dc input is obtained by connecting a chopper between dc supply and inverter. When supply is ac, dc input voltage is obtained by connecting a controlled rectifier between supply ac and inverter. A large electrolytic capacitor is connected in dc link to make inverter operation independent of rectifier or chopper and to filter out the harmonics in dc link voltage. Harmonics are reduced, low frequency harmonics are eliminated, associated losses are reduced and smooth motion is obtained at low speeds also when inverter is operated as pulse width modulated inverter. Since output voltage can be controlled by PWM, no arrangement is required for variation of input dc voltage, inverter can be directly connected when supply is dc and through a diode rectifier when supply is ac.
VSI induction motor drives V = m Vd / 2√ 2 Where m is modulation index.
CURRENT SOURCE INVERTER (CSI):
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In current source inverter, inverter behaves as current source due to the presence of large inductance in dc link. For a given speed, torque is controlled by varying dc link current by changing the inverter voltage. Therefore, when supply is ac, a controlled rectifier is connected between the supply and inverter and when supply is dc, a chopper is interposed between the supply and inverter. The maximum value of dc output voltage of fully controlled rectifier and chopper are chosen so that the motor terminal voltage saturates at rated value.
CSI induction motor drives
COMPARISON OF CSI AND VSI: The relative advantages of CSI and V SI are ●
CSI is more reliable than VSI because 1
Condition of two devices in the same leg due to commutation failure does not lead to sharp rise of current through them.
2 ●
it has inherent protection against short circuit across motor terminals
Because of large inductance in dc link and large inverter capacitors, CSI drives has higher cost, weight and volume, lower speed range and slower dynamic response.
●
The CSI drive is not suitable for multimotor drives. Hence, each motor is fed from its own inverter and rectifier. A single converter can be used to feed a number of VSI
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motor systems connected in parallel. A single VSI can similarly feed a number of motors connected in parallel.
CHAPTER 5: ENERGY SAVING WITH VFD Rising energy prices create a need for energy-efficient motor control solutions. To help reduce energy losses, process engineers are turning to variable frequency drives (or adjustable speed drives) as an alternative to fixed speed controllers and throttling devices such as dampers and valves. Advancements in drive technology, careful selection of the hardware and power system configuration, and intelligent motor control strategies produce improved operating performance, control capability and energy savings. Things to consider when choosing a motor control solution include peak-demand charges, operating at optimized efficiency, power factor, isolation transformer cost and losses, regeneration capabilities, synchronous transfer options and specialized intelligent motor control energy-saving features.
Optimize Power Usage: Centrifugal loads offer the greatest potential for energy savings by using variable frequency drives (VFDs) to control speed. Energy consumption in centrifugal fan and pump applications follows the affinity laws, which means that flow is proportional to speed, pressure is proportional to the square of speed, and horsepower is proportional to the cube of speed. That means if an application only needs 80 percent flow, the fan or pump will run at 80 percent of rated speed and only requires 50 percent of rated power. In other words, reducing speed by 20 percent requires only 50 percent of the power. ●
Beat peak-demand charges VFDs also affect peak-demand electricity prices that the utility charges to companies
that exceed a preset limit, such as when industrial motors started across-the-line draw large peaks of current. VFDs help reduce the peaks by supplying the power needed by the specific application, and gradually ramping the motor up to speed to reduce the current drawn
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●
Power Factor Power factor and how it affects displacement and harmonic distortion is another
important consideration in drive selection. Drives that are near-unity true power factor translate to reduced energy use. ●
Regeneration
Some VFD applications enable s not only to save energy, but to regenerate power, which can be routed back to the system or sold to a utility for additional revenue.. ●
Multiple Motors drive
Another way to reduce energy costs is the synchronous by method which uses only one VFD to start and synchronize multiple motors by transferring a load from one source to another by matching the voltage waveform frequency, amplitude and phase relation between the two sources. Using a VFD to start a motor, bring it up to speed and then synchronize it, causes a reduction in full-load current and optimizes the process. ●
Extra Energy-Saving Potential
Intelligent motor control solutions including high-efficiency variable frequency drives are an important part of an energy savings program. But not all drives have the same capabilities. Software features and programmability can further contribute to a drive‟s energy savings potential by reducing inrush current requirements. Programmability - s can program their VFD to adjust the total acceleration time and current limit and adjust the speed to the load requirement. Current limit on drives is normally set between 105 and 110 percent, whereas using the across-the-line starting method produces current limits of approximately 650 percent.
Department of Electrical and Electronics,SDM IT
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SGCTs – Advances in power semiconductor switches like SGCTs (symmetrical gatecommutated thyristors) are designed for high-voltage operation and ensure the lowest switching and conduction losses while maintaining a high switching frequency. Power Optimization – Power optimizing features optimize the power usage when operating fans and pumps by adjusting the required voltage to the application. This reduces losses for improved motor and drive efficiency. Communication software – Software features enable torque limit and integrated architecture through communication connectivity between the drives, starters and soft starters for greater control and optimizatio
Department of Electrical and Electronics,SDM IT
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VARIABLE FREQUENCY DRIVE
CHAPTER 6: APPLICATIONS The three major areas where drives and motor are applied are Variable torque application – When a low torque is needed at low speed and high torque at high speed. These applications are generally centrifugal loads such as fans, pumps and blowers. These are good targets of energy savings. Constant power application – When a high torque is required at low speed and low torque at high speed. It includes machine tools and traction. Constant torque application- When the same amount of torque is required at low speed as at high speed. Power is directly proportional to speed. They include conveyors, mixers, screw feeders, extruders and positive displacement pumps. Lifts – Energy saving when regenerative breaking is involved. Transport- Where all the vehicles, train, ship etc involves motor, VFDs are used for speed control.
The latest developments in technology and successful development of electronic drives (AC drives) for cage motors have resulted in the following benefits: 1
Availability of full load torque from standstill
2
Absence of torque fluctuations at low speed.
3
Ability to hold a set speed, regardless of load torque variation
4
Ability to control the rate of increase & decrease of speed
5
Dynamic response.
Department of Electrical and Electronics,SDM IT
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VARIABLE FREQUENCY DRIVE
CHAPTER 7: ADVANTAGES AND LIMITATIONS
Advantages: ●
Soft starting
●
Precise speed and torque control
●
Wide speed control torque
●
High reliability and availability
●
Low audible noise
●
Capability of speed reversal / regenerative breaking
●
Flux optimization
●
Power loss ride through
●
Energy saving
Limitations: ●
AC drives not reliable as dc drives
●
Cost of AC drives are more
●
Presence of harmonics may heat the motor
Department of Electrical and Electronics,SDM IT
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VARIABLE FREQUENCY DRIVE
●
Separate filter circuits may be required to reduce harmonics
CONCLUSIONS: For precise process control and wide speed range, to provide good running and transient performances variable frequency drives are natural choice. The report describes functioning of such a drive. It is very essential to know the applications of them. The consumer is using them enormous. Hence, this paper concludes that variable frequency drive plays an in important role in speed control of ac electric motors and also VFDs are simple, reduced thermal stress on motor, most energy efficient, reduced noise levels, energy savings, soft starting and stopping are the key features of the VFD.
Department of Electrical and Electronics,SDM IT
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VARIABLE FREQUENCY DRIVE
BIBLIOGRAPHY: 1
Gopal k.Dubey, Fundamentals of electrical drives, , Narosa Publishing house, II edition
2 3 4
Bimal K Bose, Modern power electronics and AC drives M. H Rashid , Power electronics, Pearson India, II edition Bimal K Bose,The past, present and future of Power Electronics, IEEE industrial magazine vol.3
5
Marcel Dekker ,Solid state AC motor controls, ,New York
6
Carrier drive technology Syracus, Variable frequency drive technology, New York
7
www.wikipedia.org
8
nptel.iitk.ac.in
9
www.ab.com
Department of Electrical and Electronics,SDM IT
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